(1) Field of the Invention
The invention relates to processes for packaging MEMS devices, and MEMS packages produced using the method, and more particularly, to a method of packaging MEMS devices using lead-frame packages.
(2) Description of the Related Art
Micro-electro-mechanical systems (MEMS) devices are known which convert physical phenomena, such as pressure, acceleration, sound, or light, into electrical signals. Each type of MEMS device interacts with the world in a different way, and demands custom or at least semi-custom packaging solutions. So-called system-in-package techniques attempt to form an entire microsystem—which could include a microprocessor, communications components, actuators and sensors—within a single package. However, packaging of a MEMS device is totally different from packaging an integrated circuit device. MEMS devices are categorically different from ICs despite sharing some fundamental processing technologies. Packaging is the biggest challenge for commercializing most MEMS devices. The term “MEMS package” is used in this document to imply a package including at least one MEMS device.
The packaging requirements for a MEMS microphone or acoustic sensor are complex because the devices need to have an open path to receive sound pressure and yet the devices need to be protected from external environmental hazards like particles, dust, corrosive gases and humidity. In addition, the device must have an acoustic chamber to be functional. A sufficient acoustic chamber is an essential requirement to guarantee the desired performance of any microphone/acoustic sensor. An insufficient acoustic chamber will damp the diaphragm movement and increase the acoustic noise. In considerations of packaging a MEMS device, it is also desired to avoid expensive processes and costly substrate and assembly materials.
For example, FIG. 1 illustrates a MEMS package as disclosed in U.S. Pat. No. 7,375,416 (Retuta et al). This is a lead-frame method used to produce a multi-row semiconductor package. Surface Mount Technology (SMT) pads are used for external connections. However, this lead-frame package has plastic 307 molded over the semiconductor IC devices and is not suitable for MEMS devices with movable structures on their top surfaces.
FIG. 2 illustrates a MEMS package as disclosed in U.S. Pat. No. 7,166,911 (Karpman et al). A MEMS inertial sensor 16 is secured within a pre-molded type package formed from a low moisture permeable molding material. Pins 20 are provided for external connections. The premolded package does not have an inlet to provide environmental communication with the MEMS sensor and the pre-molded plastic sidewalls of the package do not offer electromagnetic interference (EMI) shielding.
U.S. Patent Application 2008/0150104 (Zimmerman et al) discloses a premolded lead-frame package having an opening 407 to the MEMS sensor 403 and having EMI shielding in a conductive cover 401, shown in FIG. 3. However, such a package cannot provide a sufficient acoustic chamber for MEMS acoustic element 403 when the sound comes from inlet 402.
U.S. Pat. No. 7,202,552 (Wang et al) teaches methods of using flexible printed circuit boards and folding processes to make MEMS packages, as shown in FIG. 4. An opening 11 for sound energy and SMT pads 64 for external connection are shown. However, the flexible substrate material is not easy to handle in the assembly process and is expensive compared with lead-frame substrates.
U.S. Pat. No. 7,242,089 (Minervini) teaches that an FR-4 substrate is preferred over a plastic body/lead-frame design because of its flexibility in changing product design. But, an FR-4 substrate is still a more expensive alternative compared to a lead-frame package. U.S. Pat. No. 7,419,853 (Kuhmann et al) shows leads extending along the sides of a silicon cap structure.